The experiments described in this proposal are aimed at characterizing the mechanisms that regulate the size of individual cells as well as the size of whole organs. We have conducted a genetic screen in the fruit fly, Drosophila, and identified mutations in at least sixteen different genes that result either in increased cell size or an increase in the overall size of the eye. In the previous funding period, we showed that the Drosophila orthologs of the tuberous sclerosis genes, function to restrict cell growth in vivo. We have also identified and characterized several novel regulators of cell and tissue growth including Archipelago as well as a complex composed of the Salvador, Warts and Hippo proteins. In this application, experiments are proposed to characterize several pathways that regulate tissue growth but that are still poorly understood. Our laboratory has recently shown that the HMG-box protein Capicua functions downstream of the EGF receptor and Ras to regulate cell growth. Specific Aim 1 details experiments that will help define the precise role of Capicua in this pathway. First, experiments are proposed that wil investigate the mechansim by which activtton of the Egfr/Ras pathway downregulates Capicua levels. Second, using oligonucleotide microarrays and chromatin immunopreciptiation, we will identify genes whose expression is directly regulated by Capicua. Specific Aim 2 proposes an analysis of mutations that increase tissue growth by a non-cellautonomous mechanism. In these mutants, the mutant cells grow poorly but their presence leads to the overgrowth of adjacent wild-type tissue. Our experiments will help define the mechansim by which this overgrowth occurs by characterizing the nature of the growth promoting signals produced by the mutant cells. Together, these studies will help us understand how growth is regulated in vivo during the normal development of an organism. Our findings will also be relevant for understanding the perturbed growth that occurs in human cancers.